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Fair Value Measurements - Additional Information (Detail) - USD ($)
Mar. 31, 2018
Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Other current liabilities $ 7,000,000  
Other long-term liabilities 6,672,000 $ 643,000
Transfers from level 1 to level 2 0  
Transfers from level 2 to level 1 0  
Merck, Sharp & Dohme Corp.    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Other current liabilities 7,000,000  
Merck, Sharp & Dohme Corp. | Fair Value, Inputs, Level 3    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Other long-term liabilities $ 5,800,000